The present invention relates generally to methods of forming polycrystalline thin films and, more particularly, to methods using laser annealing and lateral crystallization.
Polycrystalline silicon thin films are used to form thin-film transistors (TFTs) for pixel-switching elements and other integrated circuits that are simultaneously fabricated on display substrates. These thin films and TFTs can provide for the fabrication of integrated circuits on various substrates, for example, glass, plastic, or metal. These thin films and TFTs may also be used for non-display applications as well. Possible non-display applications include, sensors, ASICS, memory modules, or printer heads, for example.
Polycrystalline silicon, also known as poly-Si, films may be produced by crystallizing amorphous silicon, or microcrystalline silicon. Quality poly-Si films can be produced using lateral growth processes, also referred to as lateral crystallization. Quality poly-Si films can then be used to produce high performance poly-Si TFTs. The quality of the films and the resulting TFTs depends to a great extent on the crystal characteristics. Laser induced lateral crystallization, which uses an excimer laser to crystallize amorphous silicon such that the crystal grows in a lateral direction, has been used to produce quality poly-Si films. By moving the laser and sequentially exposing adjacent regions it is possible to form polycrystalline films having long crystal grains oriented in the scanning direction.
Despite the success of laser-induced lateral crystallization, and related crystallization techniques, in producing quality poly-Si materials problem areas persist. There is a continued lack of uniformity in the material characteristics of the films produced. The lack of uniformity results, in part, from the formation of sub-boundaries along the direction of lateral growth. The sub-boundaries generate trap states within the active layer, which may modulate device operation resulting in non-uniformity in device characteristics, including threshold voltage. Another problem is related to the lateral growth length (LGL), which is the distance that the crystal grows laterally for each laser shot. The LGL is currently limited to between approximately less than 3-5 μm. This LGL limitation also affects techniques that employ sequential crystallization by scanning, because the moving pitch (p) between sequential laser shots is even more restricted in that the moving pitch should be less than the LGL, (p<LGL). Crystallization over lengths of between about 30 μm and 100 μm, which is desired for device fabrication, requires many shots, which translates to longer process times and reduced productivity.